Synchronous counter driven encryption techniques for full duplex digital wireless communication systems are well known in the art. Such systems normally employ separate encryption and decryption devices in each communication unit for encrypting and decrypting a transmit and receive path, respectively. The use of separate encryption and decryption devices in each communication unit adds to the cost and size of the communication unit, as well as complicating the synchronization of the two devices.
In order to maintain synchronization between two linked communication units, conventional systems must transmit an encryption synchronization signal (E-sync) along with the encrypted information. Transmitting the E-sync is not a problem when the information is stored data, which can be interrupted without concern. On the other hand, transmitting the E-sync in a voice communication system becomes somewhat more difficult, because the voice information is continuous and cannot be interrupted periodically for an E-sync transmission without noticeable gaps or noise bursts in the received voice audio signal.
Some conventional voice encryption systems "steal" bits periodically from the voice information and use the stolen bits for the transmission of the E-sync, the theory being that if the bits are stolen only infrequently, their absence will not seriously degrade the voice audio. Still, missed bits do degrade the voice audio quality somewhat, causing encryption systems based on the use of stolen voice bits not to rate as well in subjective tests of audio quality while in an encrypting mode as they rate in an unencrypted, i.e., "clear" mode.
Yet another problem with conventional encryption techniques used in wireless communication systems that can hand-off a portable communication unit (PCU) from one fixed communication unit (FCU) to another is associated with the hand-off procedure. The problem occurs because when two communication units are linked, the encryption device for the transmit path of each of the two communication units supplies the E-sync signal for the decryption device in the corresponding receive path of the other communication unit. Thus, after a hand-off to a new FCU, the encryption synchronization is lost for a period of time required to resynchronize the decryption device in the PCU with the new E-sync from the new FCU, and the decryption device in the new FCU with the E-sync from the PCU.
As the loss of encryption synchronization would cause the loss of all communicated information during the resynchronization period following the hand-off, conventional encryption systems for sending continuous information such as voice must revert to the clear mode prior to each hand-off, followed by a return to the encrypted mode after sufficient time has elapsed for encryption synchronization to be reestablished. This of course implies that each hand-off is accompanied by a brief period in which the security of the transmitted information is compromised.
Consequently, what is needed is an encryption technique that overcomes the aforementioned problems of conventional encryption techniques. That is to say, an encryption technique that can continue operating in the encrypted mode throughout a hand-off with no loss of information is needed. An encryption technique that does not degrade the voice quality is needed. Furthermore, an encryption technique that can be built with a lower cost and a smaller size than allowed by conventional encryption techniques is needed.